Decrease in electrical resistivity on depletion of islands of mobility during aging of a bulk metal glass

Abstract

The effect of structural relaxation on electrical resistivity, ρ, of strain-free ZrTiCuNiBe bulk metallic glass was studied during isothermal aging at several temperatures, Ts. Since cooling of a liquid metal increases its resistivity ρ, one expects ρ to increase on aging toward ρ at T = T. Instead, ρ decreased non-exponentially with the aging time. The activation energy of aging kinetics is 189 kJ mol, which is higher than the activation energy of the Johari-Goldstein (JG) relaxation. After considering the sample's contraction, phase separation, and crystallization as possible causes of the decrease in ρ, we attribute the decrease to depletion of islands of atomic mobility, soft spots, or static heterogeneity. Vibrations of the atoms in these local (loosely packed) regions and in the region's interfacial area contribute to electron scattering. As these deplete on aging, the contribution decreases and ρ decreases, with a concomitant decrease in macroscopic volume, enthalpy, and entropy (V, H, and S). Local regions of faster mobility also decrease on cooling as V, H, and S of a liquid decrease, but structure fluctuations dominate electron scattering of a liquid metal and ρ increases effectively according to the Ziman-Nagel theory for a homogenously disordered structure. Whether depletion of such local regions initiates the structural relaxation of a glass, or vice versa, may be resolved by finding a glass that physically ages but shows no JG relaxation.

Source of JG-Relaxation in the Entropy of Glass

“…Eshelby inclusions appear to define mathematically the "islands of mobility" or "soft spots" in a glass structure, but it is not clear how such inclusions would be sustained in an ultraviscous melt in which the JG-process occurs, [6][7][8][9][10]15,16 how their volume would decrease on cooling, and how would they deplete during isothermal aging of a glass that decreases Δε JG , a subject recently discussed in the context of change in electrical resistivity on aging of a metallic glass. 127 Obviously, further studies are needed not only for determining the advantage of using the Eshelby concept, but also for resolving whether the decrease in H, S, and V on aging of a glass can be attributed to depletion of the islands of mobility during aging or to depletion of the atomic level stresses in a homogeneous structure of a glass or to both. It is obvious that the concepts of Eshelby stress and atomic-level stresses, although valuable in a discussion of the glassy state, would not be applicable to the structure of orientational glasses that mimic the thermodynamic and dielectric relaxation features of polymers and molecular glasses.…”

Long-Range Mass Transport during Structural Transitions in Metallic Glass-Forming Melts

“…However, taking the very large diffusion length into account, we cannot exclude here the possibility of involving different melt compositions, e.g., originated from chemical decomposition. This should be experimentally detectable via, e.g., small angle scattering techniques or resistivity measurements [28,29]. Although particularly in the case Vit1, spinodal decomposition in the glass has been reported, the results are still controversial and are observed at much lower temperatures [30][31][32].…”